As methods for quenching an alloy from a molten state so as to continuously produce strip or wire, the centrifugal quenching method, single roll method, twin roll method, etc. are known. These methods eject molten metal from an orifice etc. to the inner circumference or outer circumference of a metal drum rotating at a high speed so as to rapidly solidify the molten metal and produce strip or wire. Further, by suitably selecting the alloy composition, an amorphous alloy similar to liquid metal can be obtained and a material superior in magnetic properties or mechanical properties can be produced.
This amorphous alloy strip is considered promising as an industrial material in numerous applications due to its superior characteristics. In particular, for applications for iron core materials for power transformers, high frequency transformers, etc., due to the low watt loss, high saturated flux density and permeance, and other reasons, Fe-based amorphous alloy strip, for example, Fe—B—Si-based strip, is employed.
As the technical issues involved in use of amorphous alloy strip as the material for iron cores of power transformers, high frequency transformers, etc., there are the greater amount of material used when producing the transformers, for example, the iron core and copper wire, and the higher production costs compared with use of silicon steel plate. This is due to the fact that most amorphous alloy strips are small in saturated magnetizing force and therefore the design flux density of the transformer has to be lowered. As a result, the cross-sectional area of the iron core becomes larger.
Therefore, various studies have been conducted on improving the flux density of amorphous alloy strip. For example, Japanese Patent Publication (A) No. 03-264654 proposes an amorphous alloy strip comprised of a composition of Fe80B20 wherein a saturated flux density of 1.57 to 1.61 T (Tesla) is obtained and Si and P are added to improve the embrittlement temperature and ductility. Further, in Japanese Patent Publication (A) No. 03-500668, a high flux density was confirmed due to the addition of Co in an Fe—B—Si—C-based amorphous alloy strip, but Co is an expensive element, so there are cost difficulties. Therefore, as a system of ingredients able to realize high flux density without use of Co, Hatta et al.: JEEEE Trans. Magnetics MAG-14 (1978) 1013 introduces an Fe—B—C-based amorphous alloy strip. It is reported that with this system of ingredients, a 1.78 T saturated flux density is achieved, but there are the problems that the watt loss is poor compared with an Fe—B—Si—C-based amorphous alloy strip and that the heat stability, as represented by the stability of the magnetic characteristics at the time of annealing and the time of transformer operation, is low in level.
Further, Japanese Patent Publication (A) No. 09-95760 proposes that the allowable amounts of the contents of S, Mn, and other impurity elements be increased by the addition of a slight amount of P: 0.008 to 0.1 wt %, but the effect on the heat stability and workability (brittleness) accompanying the addition of P is not evaluated. Further, Japanese Patent Publication (A) No. 62-74050 proposes adding N to an amorphous alloy strip containing Cr so as to raise the hardness of the strip and improve the maximum permeance and watt loss, but the problems of heat stability and workability are still not solved.
Further, when producing Fe—B—Si-based amorphous strip, it had been thought that impurities degraded the watt loss etc., so in the past alloy materials with impurities kept extremely low had been used. That is, as the iron source, electrolytic iron had been used.
As the specifically suppressed impurities, for example, there are P and S. Japanese Patent Publication (A) No. 59-16947 limited P to 0.015 wt % or less and S to 0.02 wt % or less. This publication describes P as an element degrading the watt loss and further S as an element promoting brittleness. The composition is prescribed as Fe: 86 to 95 wt %, B: 2 to 4 wt %, Si: 0 to 11 wt %, and C: 0 to 1.5 wt %. If converting these to atm %, wide ranges of Fe: 65.9 to 85.4 atm %, B: 8.3 to 17.6 atm %, Si: 0 to 18.3 atm %, and C: 0 to 6.1 atm % are taken.
Further, Japanese Patent Publication (A) No. 57-137451 shows the maximum allowable amounts of various impurity elements in FeSiB-based amorphous strip. For example, it prescribes P: 0.008 atm % or less, Mn: 0.12 atm % or less, and S: 0.02 atm % or less. This publication prescribes Fe: over 78.5 atm % to less than 80 atm %, B: 13 atm % to 16 atm %, and Si: 5 atm % to 10 atm %, so if converting the maximum allowable amounts of the impurity elements into wt %, they become P: 0.0053 wt % or less, Mn: 0.14 wt % or less, and S: 0.0136 wt % or less. This publication as well considers the impurity elements to be elements degrading the characteristics.
The allowable amounts of the impurities in the case of producing amorphous alloy strip are considerably small as shown in these Japanese Patent Publication (A) No. 59-16947 and Japanese Patent Publication (A) No. 57-137451, so it had been thought difficult to use the steels produced by usual steelmaking processes from iron ore for the iron source of amorphous alloy strip. The reason is that these iron sources include more than the allowable amounts of impurities.
That is, in the past, the allowable amounts of the impurity elements were considerably low, so electrolytic iron and other high purity iron sources had to be used. High purity iron sources are expensive, so the strip alloy cost became high. This became a factor raising the cost of production of strip. To promote the broader use of strip as an industrial material, the production cost has to be reduced. For this purpose, it has been strongly desired to reduce the strip alloy costs. Further, in the past, the characteristics varied in the same lot. This became a factor lowering the yield and raising the production costs.
Therefore, the applicant previously proposed in Japanese Patent Publication (A) No. 09-202946 an alloy strip exhibiting excellent characteristics without using electrolytic iron or another high purity iron source as the material for the strip alloy, that is, even if using an inexpensive iron source. That is, they proposed an Fe-based amorphous alloy strip consisting of strip comprised of main elements of Fe, B, Si, and C and impurities characterized in that the composition of the main elements is expressed by FeaBbSicCd, where a, b, c, and d are, by atm %, 80<a≦82, 14≦b≦16, 2≦c<5, and 0.02≦d≦4, and by containing as impurities, by wt %, P: 0.008% to 0.1%, Mn: 0.15% to 0.5%, and S: 0.004% to 0.05%.
This invention was made based on the finding that if including a slight amount of P, even if Mn, S, and other impurities are included in greater amounts than the past, the characteristics of the strip will not degrade, so use of a low grade iron source containing a certain extent of impurities becomes possible. In general, low grade iron sources are inexpensive, so the strip alloy costs can be reduced.
Further, in an ingredient system containing slight amounts of P, Mn, and S, by limiting the amounts of Fe, B, Si, and C to limited narrow ranges, strip improved in watt loss and with less variation in characteristics in the same lot can be stably obtained. According to this invention, an improvement in the yield can also be simultaneously realized.
Further, Japanese Patent Publication (A) No. 2001-279387 proposes a matrix alloy for producing rapidly cooled and solidified strip containing P, Mn, and S at the levels shown in Japanese Patent Publication (A) No. 09-95760 and containing as component elements, in addition to Fe, B, and Si, Ti, Zr, V, Nb, Cr, Mo, Co, Ni, and Cu, by atm %, 0.1 to 30%. According to this invention, utilization of a broader range of low grade iron sources is realized.
As explained above, use of a low grade iron source containing slight amounts of P, Mn, and S is made possible, so an inexpensive iron source can be used and the cost of the strip alloy can be reduced. Further, by optimizing the range of main elements in the ingredient system containing slight amounts of these impurities, achievement of a stable watt loss characteristic in a lot can be realized. However, there is a great need for improvement of the characteristics of Fe-based amorphous alloy strips. Further improvement of the watt loss is being demanded. In the inventions proposed in the previously mentioned Japanese Patent Publication (A) No. 09-202946 and Japanese Patent Publication (A) No. 2001-279387, the watt loss could for example be improved to 0.12 W/kg or less in terms of the watt loss W13/50 (watt loss at flux density of 1.3 T and frequency of 50 Hz) measured by a single sheet, but it was extremely difficult to stably lower this to 0.10 W/kg or less.